Throttle position sensor/circuitry checking w/resp. to poor fuel econ.

benswann at benswann at
Wed Sep 10 04:40:29 EDT 2003


I was going to run a parallel wire outside the harness just for a check.  I
betting well have the same voltage - maybe not.  I don't think it is wiring.
It is like 3.5 V comingdirectly out of ECU rather than expected 5V.  According
to wiring diagram it is one straight piece of wire from ECU harness/connector
Pin ( forget off hand which no.) to TPS pin no. 3 - perhap there is a connector
or even a resistor somewhere in between not documented.

Coincidentally the other car had similarly low voltage.  On both cars, Pins 1-2
was ~5V  as supposed to be.  It also exibited some throttle flakyness and poor
fuel econ. based on my test drive.

I'm wondering if this is a Bentley descrepancy - rare, but seen them before.

Also, does my TPS not reaching 3700ohm when going WOT indicate it is marginal
or bad - someone else had same question recently.


> I would check the wire that carries the voltage to the pins for excess
> resistance. Just b/c the wires look OK doesn't mean they're fine. Corrosion can
> affect copper a great deal especially under the hood where the insulation might
> melt due to heat. Also, you might want to check your multimeter for accuracy by
> testing it w/ some voltage you know is correct, such as the battery, etc. To
> measure the resistance of the wires going to the TPS, first get a length of wire
> or alligator clips to reach from the beginning and end points (ie. ECU and TPS
> connector). Measure the resistance of the additional wiring alone before
> measuring the wires from the harness. This will value will be subtracted to
> zero-out any resistance in the testing leads and additional alligator clips,
> much like zeroing-out the mass of a container to measure the mass of a liquid.
> Then, make sure no power is going through the wires (otherwise you will get
> false readings) and test the resistance of both the positive and ground wires
> from the harness. Don't forget to subtract the resistance of the test leads. I
> would imagine the resistance should be less than half an ohm, but the Bentley
> may have a spec. At about 0.5 ohms the wire will drop the voltage from 5.3V to
> 3.3V, assuming a current of 250 mA. The more the current flowing through the
> wires, the larger the voltage drop and the same w/ resistance. This is from
> Ohm's Law: V=IR. At least this way you can rule out the wire's fault and move
> onto other possibilities. Good luck.
> -Ameer
> >One of the suspects I've been trying to rule in/out is the throttle position
> sensor and circuitry.
> >According to Audi/Bentley Sec 24-120-2 the voltage between pins 1 and 3 must be
> 4.5 to 5.5
> >volts. On my car the reading was 3.3V. According to Bentley that means faulty
> wiring or bad ECU.

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